The invention relates to radiological cameras and systems, and particularly, to a stationary radiological camera having the capability of total isolation of positron producing events, such as the decay of certain radioisotopes.
The generation of distribution maps of radioactive materials, generally gamma emitting radioisotopes, within a patient has become a very important aspect of nuclear medicine.
A variety of cameras which are capable of producing excellent two-dimensional maps have been developed. The rectilinear scanner described by B. Cassen, L. Curtis and R. Libby, in the 1951 Volume of Nucleonics, at p. 46, uses a single collimated NaI crystal which mechanically traverses the area of interest over a patient while a count rate is generated and recorded in spatial synchrony. M. A. Bender and M. Blau have described, in the 1963 volume of Nucleonics, at p. 52 the auto fluoroscope which consists of a fixed rectangular array of discrete NaI crystals. The widely used Anger camera, H. W. Anger, Review of Scientific Instruments Vol. 29 (1958), p. 27, employs a large continuous disc of NaI which is then viewed by a dense array of photomultipliers tubes. The (x, y) coordinates of a gamma detection event in the scintillator are formed as a continuous function of the photomultiplier tubes outputs by weighted summation.
Stationary detector system, such as the Anger camera, are especially favored in applications where it is desired to image rapidly changing tracer concentrations because of their ability to spatially analyze events at high frame rates, uncompromised by the time limits imposed by a scanning process.
All of the above systems are severely limited in depth resolution. One approach to obtaining depth information is the use of a focused collimator, but, as in all systems relying on focusing, fixed focal lengths and depth of focus limit the utility. Other approaches to such tomographic imaging and improvements thereof are reviewed by G. S. Freeman "Tomographic Imaging in Nuclear Medicine", Society of Nuclear Medicine, Inc. New York, 1973. In general, it may be stated that there is a widespread clinical appreciation of the need for better depth resolution and the price that must be paid for it in terms of increased instrument complexity, imaging time, etc.
To our knowledge, only two instruments have been studied with the aim of obtaining true visualization of depth through total isolation of any desired section. In one of these a dual camera system viewing the cascade emissions of radionuclides, such as .sup.75 Se, has utilized conventional counting and coincidence counting at 90.degree. to record three dimensional activity distributions. This instrument is disclosed by W. G. Monahan and M. D. Powell in "Three Dimensional Imaging of Radionuclides Distributions by Gamma Coincidence Detection", in the aforesaid Tomographic Imaging in Nuclear Medicine.
In the second instrument, described by C. Burnham, S. Aronow and G. L. Brownell, in "New Instrumentation for Position Scanning", presented at the International Conference on Radioisotopes in Localization of Tumors, England, Sept. 25-27, 1967, a different approach involved the one-dimensional depth measurement of a positron source on a line between two probes by directly observing the time-of-flight of the two 511 keV gammas resulting from the annihilation of the positron. They were able to resolve the position of the positron source to within 12 cm. (FWHM) along the line between the probes. We have, in effect, reproduced their experiment utilizing currently available fast acting scintillators and timing circuitry and obtained a resolution of about 4.5 cm.
We have combined this just described approach with a novel one dimensional system we have developed for locating a scintillation occurring along the length of a scintillator bar by a time-of-flight technique. And we have, in effect, put these two instruments together, -- the prior art instrument capable of locating a positron annihilation event in one dimension, and our new instrument capable of locating the point of scintillation along the length of a scintillator bar, in a unique spatial arrangement such as to define a unique tetrahedron shaped volume within which the overall camera is sensitive to, and is able to uniquely locate the position of, positron emitting radionuclides.
The result is a novel three dimensional stationary camera having the capability of totally isolating the positron annihilation event which occurs almost immediately after a positron emitting radioisotope emits a positron.
Positron radiography has many special uses and advantages for certain purposes, these being fully reviewed in the aforementioned Burnham et al article.
Accordingly, it is one object of our invention to provide a novel apparatus and method for locating a scintillation point along an elongated scintillator bar.
The ultimate object of our invention is to provide a stationary three-dimensional camera system capable of total laminographic isolation of a positron producing radioactive source within a patient.
In brief, we accomplish the above object by exploiting an unorthodox tetrahedronal geometry, wherein a pair of skewed scintillator bars define a unique tetrahedron camera sensitive volume. Photomultiplier tubes are disposed at each end of both bars similar to the physical arrangement shown in FIG. 1 of the U.S. Pat. No. 3,688,113, Tomographic Radiation Sensitive Device, issued Aug. 29, 1972 to Miraldi. The four photomultipliers generate four fast timing pulses which can be processed to uniquely determine the location of the positron annihilation through time-of-flight and coincidence techniques.
We have thus invented a three-dimensional positron camera which departs from previous stationary approaches in its simplicity -- subtending a large field of view with only two detector and four photomultiplier tubes. We have also conceived and described below means for increasing the sensitivity of our camera by the square of the number of pairs of scintillating bars employed.